1. Field of the Invention
The present invention relates to an optical scanning apparatus with a unit that suppresses curvature of a scanning line, and an image forming apparatus having the optical scanning apparatus incorporated therein.
2. Description of the Related Art
FIG. 1 is a general configuration example of a scanning optical system in the optical scanning apparatus.
In FIG. 1, laser beams L are emitted from a light source 1, and coupled and formed by a coupling optical system including a coupling lens 2, a cylindrical lens 3, and an aperture 4. After being coupled, the laser beam L is deflected and reflected by a rotating polygon mirror 5, scanned in a main scanning direction, and imaged as a scanning line 9 on a surface 8 to be scanned as an image surface, in this example, on a columnar photoconductor 10 as a columnar image carrier, by a first scanning lens 6a and a second scanning lens 6b, which form an imaging optical system. The photoconductor 10 rotates about a central axis in a circumferential direction, so that an image forming surface moves in a direction vertical to the main scanning direction, optical scanning is performed by the laser beam L, and a latent image is formed by exposure.
Reference numerals 7a to 7c in FIG. 1 are folding mirrors that deflect the laser beam L from the second scanning lens 6b towards the surface 8 to be scanned.
According to conventional techniques in this field, in the scanning optical system, there has been proposed a configuration that a scanning line curvature can be corrected by providing an optical element-deforming unit (not shown) in, for example, the second scanning lens 6b, even when there are factors such as a manufacturing error of the optical scanning apparatus, for example, a metrication error in the optical element, eccentricity of an optical surface, and an installation error.
Japanese Patent Application Laid-open No. 2002-131674 discloses a technique to provide a mechanism for making a position for supporting a scanning lens movable, and inclining an optical axis of the scanning lens in a sub-scanning direction relative to an optical axis of the scanning optical system. However, the problem of the scanning line curvature is not solved.
Japanese Patent Application Laid-open No. 2004-109761 discloses a technique in which occurrence of the scanning line curvature due to a temperature change with the lapse of time is reduced by providing a lens shape-maintaining unit formed of a sheet metal, and a scanning-line curvature-correcting unit and an inclination correcting unit are provided in the scanning lens.
Japanese Patent Application Laid-open No. 2004-287380 discloses a technique in which a pressing member is provided relative to a long scanning lens, a unit that deforms the scanning lens is provided for each lens-clamping member, thereby improving adjustment tolerance, and the scanning line curvature is corrected in initial manufacturing adjustment.
Japanese Patent Application Laid-open No. 2005-62834 discloses a technique for reducing the scanning line curvature by providing a scanning line curvature-adjusting member in a so-called grazing-incidence scanning optical system, in which a plurality of beams enter with an angle different in the sub-scanning direction. However, it is only described that a refracting power in the sub-scanning direction is small outside the axis as the characteristic of the optical element, and it is not sufficient for suppressing the occurrence of the scanning line curvature only by this condition. Further, the description is only for the grazing-incidence scanning optical system.
Accordingly, a measure against the scanning line curvature due to the manufacturing error, which generally occurs in the scanning optical system, is required.
In recent years, due to developments of high-definition full-color image forming apparatuses, in the optical scanning apparatus used in this type of image forming apparatuses, it is required to reduce deviations, namely out-of-color registration, which occurs when dots in respective colors are superposed at a desired position, in addition to improvement of reproducibility of an image by downsizing a beam spot diameter.
Currently, demands for optical properties such as field curvature correction, reduction of sub-scanning lateral magnification, or downsizing of optical spots are further increasing due to demands for higher image quality. Therefore, reduction of flexibility in the sub-scanning direction in the scanning optical system is a problem in improving the image quality.
In a lens having a strong refracting power in the sub-scanning direction, however, decentering between opposite optical surfaces becomes the main cause of occurrence of the scanning line curvature. Further, in the scanning optical system formed of two scanning lenses, there is a general configuration that the refracting power in the sub-scanning direction is borne by a lens closest to the surface to be scanned. Accordingly, it has been proposed to apply a scanning-line curvature-correcting unit that corrects the scanning line curvature to a scanning lens having the refracting power in the sub-scanning direction (for example, see Japanese Patent Application Laid-open Nos. 2004-109761 and 2004-287380).
However, correction becomes difficult according to the shape of the scanning line. Therefore, it is required to provide an optical scanning apparatus that can achieve higher image quality by including an optical system having a shape capable of easy correction of the scanning line curvature.
When the scanning-line curvature-correcting unit has high precision, the scanning line curvature can be corrected effectively. However, the configuration thereof becomes complicated, thereby causing a cost increase of the apparatus itself due to an increase in the number of parts, a decrease in the productivity, and deterioration of a yield, and the apparatus itself becomes large.